The fuel assemblies of water-cooled nuclear reactors, and in particular of pressurized-water nuclear reactors, comprise a framework into which are introduced fuel rods consisting of a jacket enclosing a nuclear combustible material such as uranium or plutonium oxide in the form of sintered pellets.
The jacket made from a zirconium alloy tube must have a good resistance to corrosion under the effect of the primary fluid circulating in contact with the outer surface of the jacket.
In order to constitute the jacket of the fuel rods of the assemblies of water-cooled reactors, use is usually made of a zirconium-based alloy containing mainly tin and iron.
In order to improve the corrosion stability under irradiation of the jackets of fuel rods in the operating environment of the nuclear reactor, and thus to increase the lifetime of the fuel assemblies in the core, modifications or adjustments have been proposed to the composition of these zirconium alloys, or alternatively it has been proposed to replace these alloys containing tin, iron and chromium with alloys containing other elements such as vanadium, niobium or copper.
It has also been proposed, for example in patent application EP-A-0,212,351, to produce the jacket in the form of a duplex tube comprising a tubular inner core made from a zirconium alloy of a conventional type such as that described above, and a surface layer consisting of a cladding or a covering improving the corrosion stability of the jacket.
The zirconium alloy constituting the cladding or covering layer differs from the alloy constituting the core of the tube and contains iron and at least one of the elements vanadium, platinum and copper. This surface layer, the thickness of which represents 5 to 20% of the total thickness of the wall of the jacket, can be produced by extrusion of a billet consisting of an inner tube made from zirconium alloy of a conventional composition over which is fitted an outer tube having the composition of a surface layer.
The jacket is then rolled on a pilgrim step rolling mill to its final diameter.
More recently, there has been proposed in the patent application FR-A-89-00761 filed jointly by the companies FRAMATOME, COGEMA, CEZUS and ZIRCOTUBE, a duplex tube, the surface layer of which, having a thickness between 10 and 25% of the total thickness of the wall of the jacket, consists of a zirconium-based alloy containing tin, iron and niobium or vanadium. The tubular core of the duplex tube can be made from a conventional zirconium alloy in the case of the manufacture of the jackets for fuel rods, or from a zirconium-based alloy containing mainly niobium as the alloying element.
In all cases, it is necessary to ensure the perfect quality of the duplex tubes which are intended to constitute jackets for fuel rods, in particular in terms of the diameter of the tube, the total thickness of the jackets, the thickness of the outer cladding layer and the cohesion of the interface zone between the cladding layer and the core of the tube.
Checks must be carried out at the factory on very large quantities of tubes, the diameter of which is very small as compared to the length.
The checking of the diameter and the total thickness of the jacket can be carried out by using a conventional technique consisting in measuring the distance in the propagation times of pulse-shaped ultrasonic waves which are reflected by the outer surface and by the inner surface of the tube.
This ultrasonic checking and measuring technique, known under the name of the "pulse-echo" technique, may be adapted in order to take account of the cladding layer in the calculation of the total thickness of the jacket.
It has also been proposed to use a technique using ultrasonic waves in order to check the thickness of the cladding of a duplex tube based on zirconium alloy.
This technique, described in FR-A-2,629,586 filed in the name of the company CEZUS, employs an ultrasonic-wave check adapted to the measurement of a layer of small thickness, the acoustic properties of which are very similar to those of the core of the tube of greater thickness.
This improved technique does not, however, permit the measurement of cladding thicknesses of less than 0.4 mm, inasmuch as the industrial implementation of the method under satisfactory conditions requires the use of ultrasonic waves whose frequency does not exceed 20 MHz.
In the case of a cladding layer whose thickness lies between 80 and 100 .mu.m, which corresponds to the conditions encountered most commonly in the case of the duplex tubes used as jacket material, it would be necessary to employ ultrasonic waves at very high frequencies (for example of the order of 100 MHz), which makes it extremely difficult to apply the method in an industrial context.
Furthermore, in the case of jackets for fuel rods, the cladding layer and the tubular core of the duplex tube consist of very slightly alloyed zirconium-based alloys which have very similar acoustic properties, with the result that the coefficient of reflection of the acoustic waves at the cladding/core interface is very small (generally less than 2%). The interface echo is then very small and becomes drowned out in the acoustic and electronic noise of the ultrasonic signal.
A measurement method and apparatus have been proposed in FR-A-2,534,015 which make it possible to determine the thickness of a zirconium covering on a zirconium-alloy tube, employing the analysis and the measurement of currents induced in the cladding layer of the duplex tube, by magnetic induction, using an exciting current the frequency of which is selected as a function of the nominal thickness of the cladding or covering layer of the tube.
The frequency selected and the processing of the signals corresponding to the induced currents likewise make it possible to eliminate, to a certain degree, the measuring errors resulting from a variation in the width of the air gap between the exciting coil and the wall of the tube.
This technique, which is relatively complex to implement, does not make it possible, however, to compensate for the variations in the conductivity of the material constituting the core of the tube and the variations in the conductivity of the material constituting the cladding.
Furthermore, this technique does not make it possible independently to check the total thickness of the tube and the cohesion of the interface zone between the cladding or covering layer of the tube and the tubular core.